This invention relates to microwave mixers. More particularly, it is concerned with double balanced mixer apparatus for frequency conversion at microwave frequencies.
In order to convert a high frequency signal to a lower frequency at which signal processing is more readily accomplished, a mixer is employed. The mixer employs non-linear active devices which perform the frequency conversion function together with appropriate signal distribution networks which apply signals of desired magnitude and phase to the active devices. More particularly, radio frequency (RF) signals and local oscillator (LO) signals are applied to active frequency conversion devices by way of signal distribution networks, the output of the active devices being an intermediate frequency (IF) signal.
A double balanced mixer is a particular type of mixer which employs four active devices having non-linear characteristics. Double balanced mixers provide high isolation between the RF, LO, and IF signals and thus have reduced spurious output content. Double balanced mixers of hybrid form have been fabricated using diodes as the active devices. Although such mixers can provide a low noise figure, conversion gain is not achieved. In order to provide both conversion gain and low noise figure at microwave frequencies, GaAs field effect transistors (FET's) have been used as active devices. At low microwave frequencies GaAs FET's have been employed in the signal distribution networks. At high microwave frequencies monolithic and hybrid microwave mixers have been fabricated employing passive signal distribution networks mounted on a substrate. These microwave mixers, however, do not provide low noise figure and conversion gain over a broad band of frequencies together with small physical size to the extent which is desirable for certain applications.